Three-quarters of the world’s coral reefs may lack the ability to cope with climate change, despite previous optimistic predictions, according to a new review of coral research.

Earlier studies had demonstrated that some corals are able adapt to warmer water temperatures by forming new, additional symbiotic relationships with algae. But a new analysis of more than 400 coral species suggests that only one-quarter of them would be able to adapt in this way.

These latest findings add to already bleak predictions for the world’s coral reefs, which are also threatened by coastal pollution and acidifying oceans. Stressors such as these cause coral to lose the algae that keep it alive by supplying it with nutrients. Even a 1 degree rise in temperature can cause the death of this fragile animal. Some experts have predicted that Australia’s Great Barrier Reef will lose 95% of its living coral by 2050.

However, two studies published in 2004 offered hope that some corals had coped with changing water temperatures by hosting new types of algae. For example, the corals along the Panama coast that were able to switch from one type of Symbiodinium algae, known as clade C, to another one called clade D.

These corals survived the particularly devastating 1997-1998 El Nino event – a recurring climate occurrence that causes elevated sea temperatures of up to 5C on the longitude line that crosses Peru and Ecuador (see Corals adapt to cope with global warming).
One at a time

Tamar Goulet at the University of Mississippi in the US carried out a review of these two research papers and 41 others to try to understand what proportion of all coral species might possess an ability to switch algae.

She found the only corals documented to be able make this swap are those that can host multiple algae. And those that can host only one clade, or type, of algae at a time have no such switching ability.

Only 23% of the 442 coral species included in Goulet's research review were able to host more than one clade of algae. As a result, she suggests that less than one-quarter of coral species may have the ability to adapt to climate change by swapping symbiotic algae. Without adaptation, coral becomes bleached and dies.

However, Goulet says she does not know the division of species among the world’s coral reefs: it is possible that adaptable species of coral are more prevalent. The studies included in Goulet’s review only covered a small fraction of the 93,000 coral species known to exist.